1. Field of the Invention
The present invention relates to a process and to a device making it possible to measure the amount of impurities present in the trace state in a gas sample to be analysed, and in particular to a measurement process and device using a technique of analysis by laser absorption spectroscopy
It is especially suitable for detecting the water content or water vapour content of gases conventionally used in the field of the fabrication of microelectronic components.
2. Description of Related Art
Among volatile contaminants, moisture would appear to be the most harmful, by reason of the interactions which it can develop with the surfaces which it is liable to encounter, thus rendering it especially difficult to eliminate. Furthermore, the presence of water in reactive gases may give rise to serious damage in gas distribution networks.
There are at present several techniques for measuring the amount of moisture present in gases
Such techniques generally require the use of relatively expensive and voluminous equipment and are relatively lengthy to implement, thus greatly limiting their effectiveness.
An effective technique for detecting traces of water vapour in gases consists of the technique of analysis by infrared laser absorption spectroscopy, also known as xe2x80x9cTDLASxe2x80x9d.
According to this technique, the sample of gas to be analysed is placed in an analysis cell and is illuminated by light emitted by a diode according to a wavelength corresponding to the wavelength at which the absorption of light by the impurities is a maximum
Thus, for quantifying the water molecules present in the gas to be analysed, the laser ray is emitted according to a wavelength of 1.368xcexc.
According to the technique of analysis by laser absorption spectroscopy, the laser ray is divided into two beams, namely an analysis beam which passes through the analysis cell and a reference beam.
Each of the beams is thereafter detected with the aid of photodiodes.
The actual quantification of the water molecules, performed by means of an analyser operating according to this technique, is based on the Beer-Lambert law, the equation for which is as follows:
Iana=Irefxc3x97exp (xe2x88x92K.NnH20.L)xe2x80x83xe2x80x83(1)
in which:
NnH20 represents the concentration of molecules of impurities,
Iref represents the intensity of the reference beam on entry to the analysis cell,
Iana denotes the intensity of the analysis beam,
K denotes the molecular transmission coefficient, which depends on the type of gas and on its pressure, and
L denotes the optical path length travelled by the analysis beam through the cell.
In practice, quantification of the impurity molecules requires a prior step of calibrating the analyser so as to ascertain the value of the coefficient K, at the relevant pressure and for the type of gas analysed, the concentration of impurities being evaluated thereafter on the basis of the difference between the intensity of the analysis beam and the intensity of the reference beam.
The prior calibration step consists, for each type of gas and for a given value of pressure, in defining the zero of the analyser by analysing a dry gas and then in computing a calibration curve by carrying out metered additions of water to the dry gas.
Such calibration, which must be performed periodically, is relatively lengthy and tedious to implement. It is furthermore liable to give rise to errors in so far as it may be subject to pollution. Furthermore, the analyser must be accompanied by a specific setup for calibration, thus rendering it relatively voluminous.
The purpose of the invention is to alleviate these drawbacks and to provide a device and a process for measuring the amount of impurities in a gas sample capable of directly delivering a measurement of the concentration of impurities.
Its subject is therefore a process for measuring the amount of impurities in a gas sample filling a laser absorption spectroscopy analysis cell, consisting in calculating the value of a characteristic representative of the absorbance of the gas sample, at a given pressure, and quantifying the impurities on the basis of a predetermined law for the variation of the characteristic as a function of the amount of impurities, characterized in that the said characteristic consists of a quantity which varies linearly at constant pressure as a function of the amount of impurities, the impurities being quantified on the basis of a value of a coefficient of proportionality between the amount of impurities and the characteristic, determined on the basis of a table of variation of the said characteristic as a function of pressure, for a given amount of impurities.
The process according to the invention can furthermore comprise one or more of the following characteristics, taken in isolation or according to all the technically possible combinations:
the said characteristic consists of the ratio between, on the one hand, the difference between the luminous intensity of the light beam transmitted through the gas sample and the luminous intensity of the incident beam and, on the other hand, the intensity of the incident beam;
the light beam is emitted by means of a diode laser according to a range of wavelengths encompassing the wavelength at which the absorption of light by the said impurities is a maximum;
the light beam emitted by the diode laser is divided into a first analysis beam intended to pass through the cell and a reference beam, the luminous intensity of the incident beam being measured by measuring the intensity of the reference beam;
the table of variation of the said characteristic is computed for various types of gas to be analysed, by measuring the value of the said characteristic at various pressures, for a gas containing a predetermined amount of impurities;
the impurities comprise water or water vapour; and
the gas to be analysed is chosen from gases used in the field of the fabrication of microelectronic components, such as NH3, HCl, HBr, HF, NO, SiH4, GeH4, and perfluorocarbonated gases.
The subject of the invention is also a device for measuring the amount of impurities in a gas sample for the implementation of a process as defined above, comprising a diode laser intended for emitting a light beam according to a range of wavelengths encompassing the wavelength at which the absorption of light by the said impurities is a maximum, a light splitter facility adapted for dividing the emitted beam into an analysis beam intended to be transmitted through a laser absorption spectroscopy analysis cell filled with the gaseous sample to be analysed, and into a reference beam, photodetector means intended to receive the analysis and reference beams, and calculation means adapted for calculating the value of a characteristic varying linearly as a function of the amount of impurities and representative of the absorbance of the gas sample, on the basis of a comparison between the intensity, detected by the photodetector means, of the measurement and reference beams, and the calculation means include, stored in memory, a set of at least one table of variation of the said characteristic as a function of pressure for a given amount of impurities, with a view to the computation, on the basis of the said table, of the coefficient of proportionality to be applied to the calculated value of the said characteristic to obtain the amount of impurities.
Advantageously, the device includes, stored in memory, a set of tables each corresponding to a type of gas to be analysed.